1. Field of the Invention
The invention relates to an ink composition that is preferably used for ink jet recording, an ink jet recording method, a planographic printing plate obtained by using the ink composition, and a method for producing a planographic printing plate.
2. Description of the Related Art
Various methods are known for forming an image on a recording medium such as paper based on an image data signal, for example, an electrophotographic method, a sublimation-type thermal transfer method, a melt-type thermal transfer method, and an ink jet recording method. The electrophotographic method requires a process of forming an electrostatic latent image on a photoreceptor drum by charging and light exposure, and a system therefore becomes complicated, resulting in increased production cost. The thermal transfer method can be conducted by an inexpensive apparatus, but requires use of ink ribbons, leading to increased running cost and generation of waste.
The ink jet recording method can also be conducted by an inexpensive apparatus. Moreover, an image is directly formed by ejecting ink only to regions of a support which are to be image regions, and the ink can therefore be efficiently used, resulting in reduced running cost. Further, ink jet recording apparatuses are not noisy. Accordingly, the ink jet recording method is an excellent image forming method.
There is a demand for an ink composition, which is curable by irradiation of active radiation rays such as ultraviolet rays with high sensitivity to form high-quality images (radiation-curable ink composition), which can be used for an ink composition for ink jet recording. Improvement in sensitivity of the radiation-curable ink compositions which cure by irradiation of active radiation rays gives rise to several advantages, including increased curability of the ink compositions, which leads to reduced power consumption and reduced load applied to an active radiation ray generator, thereby lengthening the lifetime of the generator, and suppressed vaporization of uncured low-molecular weight substances and deterioration in strength of images formed. In addition, when the radiation-curable ink composition is used to form image regions of a planographic printing plate, improved strength of images due to improved sensitivity leads to increased printing durability to the image regions.
As the UV-curable ink composition, there has been proposed, for example, an ink composition comprising a combination of monomers having different functional groups among the monofunctional or multifunctional monomers (see Japanese Patent Application Laid-Open (JP-A) No. 5-214280). Further, as the radiation-curable composition, there has been proposed a composition containing multifunctional acrylate (see JP-A No. 8-41133). Such ink compositions have all excellent curability, while the film formed therefrom has insufficient flexibility.
Conventionally, so-called PS plates each having an oleophilic photosensitive resin layer on a hydrophilic support are used, and the photosensitive resin layer is image-wise exposed to light to form exposed regions having increased or decreased solubility in an alkaline developing solution, and non-image regions are dissolved in the alkaline developing solution and removed in producing planographic printing plates. However, along with recent broad dissemination of digitization technology in which image information is electronically processed, stored and output with a computer, there is a need for a new image output method that corresponds to the digitization technology in producing the planographic printing plates. In particular, methods of producing a printing plate without use of a developing solution are under study, and a method in which a planographic printing plate is directly produced by using an ink composition for ink jet recording has been proposed. In this method, a desired image, which is preferably hydrophobic, is formed by image-wise ejecting an ink on the surface of a support, which is preferably hydrophilic, in an ink jet recording manner and irradiating the ink with active radiation rays to cure the ink.
In order to form image regions of a planographic printing plate, it is preferred that ink droplets ejected onto on the support cure rapidly without bleeding, and that the cured image regions have excellent strength and strong adhesiveness to the support, and that when the planographic printing plate is being loaded in a printing machine, the image regions conform to the deformation of the support without generating damage such as cracks. Accordingly, there is currently a need for an ink composition suitable for such applications.